<style>
    div {
        width: 200px;
        height: 200px;
        border: 5px dotted pink;
        background-repeat: no-repeat;
        display: inline-block;
    }

    .bg {
        background-image: url(img/bg_flower.gif), url(img/bg_flower_2.gif);
    }

    .bg2 {
        background-image: url(img/bg_flower_2.gif), url(img/bg_flower.gif);
    }
</style>
</head>
<body>
<p>两张背景图：尺寸一样，第一张透明，第二张不透明
<p>
    <img src="img/bg_flower.gif"/><img src="img/bg_flower_2.gif"/>
<div class="bg">
</div>
<div class="bg2">
</div>
</body>